I have the same problem and I can't find out if it's linux or slirp related.
I am using SLiRP on a shell account either in CSLIP or in PPP mode. While in
CSLIP mode I can upload small files, sendimg email over smtp works too and
so does dcc send in irc.
But when I start it in PPP mode, I am not able to send almost *anything* out
from my host except SMTP.
Interactive sessions work in both CSLIP and PPP mode. Probably related to
the small amount of data to be sent. Downloading works at top speeds too in
any mode with any protocol (ftp, http, dcc).
I don't think it's a modem handshake problem, because I can upload at normal
speeds using minicom with z-modem (to the same host I am running SLiRP on).
Andi